1. Field of the Invention
The present invention relates to built-in antennas for radio communication terminals used for portable telephones and portable terminals, etc., and especially relates to high-gain built-in antennas for radio communication terminals capable of diversity reception with less influences of the human body during communication of a radio apparatus.
2. Description of the Related Art
A conventional built-in antenna used for portable radio apparatuses has a configuration as shown in FIG. 1. FIG. 1 is a schematic drawing that shows the configuration of a conventional built-in antenna used for radio communication terminals. Each element shown in said figure is incorporated in a cabinet of the radio communication terminal, but a general view of the radio communication terminal is omitted here to simplify the explanation. As shown in said figure, the conventional radio communication terminal is provided with tabular reverse F type antenna 7 and bottom board 1. X, Y and Z indicate their respective coordinate axes.
The built-in antenna above is also used as a diversity antenna that copes effectively with variations in the strength of the received electric field due to radio wave multi-passes. FIG. 2 is a schematic drawing showing the configuration of a diversity antenna used for conventional radio communication apparatuses. As shown in FIG. 2, it has a configuration with mono-pole antenna 3 as an external antenna in addition to conventional tabular reverse F type antenna 7 above. Diversity reception is performed through two antennas, tabular reverse F type antenna 7 which is an internal antenna and mono-pole antenna 3 which is an external antenna, providing stable communications.
The tabular reverse F type antenna with the conventional configuration shown in FIG. 1 operates as an exciter that excites the radio apparatus bottom board rather than as an antenna. Therefore, an antenna current flows in the radio apparatus bottom board and the radio apparatus bottom board controls the antenna. FIG. 3 and FIG. 4 show measured values of directivity at 800 MHz for a radio apparatus bottom board of 125 mm.times.35 mm in size. FIG. 3 shows directivity of the horizontal plane (X-Y plane) in a free space. FIG. 3 shows almost no directivity because the radio apparatus bottom board operates as an antenna. Therefore, during communication of the radio apparatus as shown in FIG. 5, electromagnetic waves are also emitted uniformly toward the human body. FIG. 4 shows the directivity of the horizontal plane (X-Y plane) during communication of the radio apparatus as shown in FIG. 5. FIG. 4 shows that there is a problem of gain reduction due to influences of the human body.
When a portable radio apparatus is communicating, it is generally tilted approximately 60 degrees with respect to the vertical direction. That is, since the portable radio apparatus is used at an angle of .alpha. degrees (approximately 60 degrees) with respect to the human body during communication as shown in FIG. 5, the polarization plane of a base station antenna differs by a degrees (approximately 60 degrees) from that of the portable radio apparatus antenna, resulting in a problem that the gain is reduced due to a mismatch of the polarization plane during transmission/reception to/from the base station.
In the diversity antenna with the conventional configuration shown in FIG. 2, if tabular reverse F type antenna 7 operates as one antenna element that makes up the diversity antenna, the antenna has the same problem as that described above.
As shown above, since the conventional built-in antenna for radio communication terminals has almost no directivity within the horizontal plane, it also emits electromagnetic waves uniformly toward the human body, having the problem that the gain is reduced by influences of the human body. Therefore, how to eliminate influences of the human body is a problem for the built-in antenna for radio communication terminals. Furthermore, since the radio apparatus is used at an angle of approximately 60 degrees with respect to the human body during communication, the polarization plane of transmission/reception to/from the base station differs by approximately 60 degrees, having the problem of a gain reduction. The question is how to match its plane of polarization with that of the base station. Furthermore, in a diversity antenna for portable radio apparatuses, if the tabular reverse F type antenna above operates as one antenna element that makes up the diversity antenna, it has the same problem as that shown above. The present invention is intended to solve these problems.